1. Technical Field
The present disclosure relates to the field of energy storage. In particular, the present disclosure is directed to an energy storage device that includes a pressure chamber containing a porous material that adsorbs air.
2. Description of the Related Art
Compressed air energy storage is commonly known by its acronym “CAES.” In some CAES devices, the air compressor is driven by an electric motor, and subsequently used to drive an air motor or turbine connected to an electromagnetic generator, thereby forming the functional equivalent of an electrochemical battery. If the charge-discharge cycle is carried out slowly enough to be approximately isothermal, meaning that the heat generated by compression dissipates without raising the temperature of the air appreciably during compression, and the heat drawn in from the environment likewise keeps the air from cooling appreciably during expansion, this form of electricity storage can have good efficiency.
CAES systems can also be engineered to have higher reliability, lower maintenance and longer operating lifetimes than chemical batteries, and their cost can be comparable to battery-based systems providing that an inexpensive means of storing the compressed air is available. Unfortunately, the high cost, weight and large size of manufactured pressure vessels in which to store the air, such as steel tanks, prevents CAES devices from competing with batteries in all of their usual applications.
To date CAES has been used for three commercial purposes. The first and most widespread use is not as a means of energy storage per se, but to power pneumatic tools and machines in shops and factories. Pneumatic tools have higher weight-to-power ratios than electrically powered tools, and the small electric motors in such tools also tend to be inefficient compared to the larger motors that drive air compressors. The compressed air is stored in a tank big enough to serve as a buffer and ensure that the pressure in the system stays constant. The overall efficiency of these systems is limited by the fact that they discard the heat of compression and do not reheat the air during its rapid expansion. This inefficiency is limited by using modest pressures, usually less than ten atmospheres, which also reduces the capital costs of such CAES systems.
The second use of CAES is for temporary backup power to keep essential machinery running in the event of a power failure, for example in computer data centers or hospitals. In such cases floor space is at a premium, necessitating the use of pressures of a hundred or more atmospheres to attain a relatively high energy density, but the cost of the high-pressure steel storage tanks for the compressed air is justified by the high reliability of the system and the high power it can immediately deliver in the event of a power failure. Subsequently a longer-term backup system like a diesel generator can be brought online if need be. Although the same functionality could be obtained from electrochemical batteries, a battery system that could deliver enough power would also have to store more energy than was needed while waiting for the long-term backup system to come online, making batteries a relatively expensive solution. A CAES system also requires less maintenance, has a longer lifetime, and does not have the disposal costs associated with environmentally hazardous chemicals. Other such short-term backup power solutions include supercapacitors and flywheels, which are likewise relatively costly.
The third commercial use to which CAES has been put is to lower the cost of generating and/or distributing electric power by utility companies. This can be done in several ways, the most common of which is to enhance central generation capacity. Large central power plants such as coal and nuclear are expensive to stop and start, while smaller plants such as gas-fired turbines are readily turned off and on but are comparatively expensive to operate. Hence, if the energy from large plants can be stored when demand is low and used to produce electricity when demand is high, the need to install and operate small peak-load plants can be reduced, thereby also reducing the average or “levelized” cost of producing electricity.